The present invention relates to a method of manufacturing an organic electrolyte electrochemical cell of structure that is unitary, i.e. in with the various component elements are connected together so as to form a single whole.
Organic electrolyte cells, in particular lithium cells are presently the subject of rapid and major development because of the high energy density they make available and because of their long lifetime. They are particularly suitable for use in objects of small dimensions.
Traditional cells are made up of solid electrodes sandwiching a porous separator containing liquid electrolyte. Because of the risk of leakage, such cells are ill-suited to consumer portable equipment.
As a result cells have been proposed that include an electrolyte that is made solid by being immobilized in a polymer matrix. However, over time electrical contact between the various components of the generator degrades, particularly if gas is given off. In order to limit that phenomenon, the cell is held in a compressed state by a rigid metal container, thereby increasing the weight of the cell. Attempts have therefore been made to interconnect the various components of the generator so that electrical contact between them is guaranteed under all circumstances.
One method of making an electrochemical cell in which the various layers are bonded together is proposed in U.S. Pat. No. 5,540,741. To constitute a first electrode, a paste is deposited on a conductive support, the paste being formed of an electrochemically active material and a polymer solution. After it has dried, the electrode is covered in a layer of a solution of a plasticizer and a copolymer of a vinylidene fluoride and of hexafluoropropylene (VDF-HFP) to form the separator. The resulting assembly is covered in a second electrode constituted by a sheet of lithium made elsewhere. Pressing is performed at a temperature of at least 150xc2x0 C., causing the copolymer to melt completely and thus leading to a non-porous material. The plasticizer is subsequently extracted by means of a solvent that is inert relative to the polymer. On being used, the cell is impregnated by the electrolyte which occupies the void left between the polymer chains by removal of the plasticizer.
That method suffers from the drawback of giving rise to large changes in dimensions during manufacture of the cell. During extraction, the structure collapses, thus leading to a dense material. Consequently impregnation with the electrolyte takes place slowly. Thereafter impregnation leads to the VDF-HFP copolymer swelling in the presence of the electrolyte solvent. Such changes in dimensions give rise to tightening and unsticking phenomena that are harmful to the electrical continuity of the resulting cell.
An object of the present invention is to provide a method of making an electrochemical cell of unitary structure which minimizes variations in dimensions.
The present invention provides a method of manufacturing an organic electrolyte electrochemical cell comprising at least one electrochemical couple made up of two electrodes sandwiching a solid film of porous polymer containing said electrolyte, each electrode comprising a porous layer containing an electrochemically active material and a binder, the method comprising the following steps:
a polymer is put into solution in a solvent;
said solution is spread in the form of a film on a support;
said film of solution is immersed in a volatile non-solvent that is miscible with said solvent in order to precipitate said polymer;
said polymer film is dried to eliminate said non-solvent; and
said couple made up of said polymer film placed between said electrodes and in contact therewith, and impregnated with said electrolyte, is pressed while being heated to a temperature less than or equal to the temperature at which said polymer film starts to melt so as to obtain incomplete melting of said polymer, said electrodes becoming uneeparable after cooling.
The invention presents numerous advantages over known methods. Because the polymer film is impregnated with electrolyte, its porosity is conserved during the sticking step. The mean size of the pores lies in the range 0.1 xcexcm to 1 xcexcm. The porous volume is large, constituting 30% to 95% of the volume of the film.
The pressing temperature is selected so that melting of the polymer film remains incomplete and restricted mainly to the surface. The film adheres on the electrodes as soon as the sticking surface has softened sufficiently Consequently, dimensions do not change during sticking, and the porous volume is not significantly altered. In addition, when electrolyte impregnation takes place after sticking, the film is observed to swell, and this is avoided by the method of the invention.
In an implementation of the method of the invention, the support used is an inert support.
The polymer film has good mechanical strength since the polymer itself contains less than 30% of the electrolyte solvent, with the major portion being contained in the pores.
In a first variant, said polymer film and each of said electrodes is impregnated with said electrolyte, and then said polymer film is placed between electrodes and in contact therewith to form said couple.
In a second variant, said polymer film is placed between said electrodes and in contact therewith to form said couple, and said couple is then impregnated with said electrolyte.
In another implementation of the method of the invention, at least one of said electrodes is used as the support as follows:
said solution is spread on the surface of said porous layer of one of said electrodes in the form of a film;
said film is immersed in a volatile non-solvent that is miscible with said solvent;
said film is dried to eliminate said non-solvent;
said electrode including said film is impregnated with said electrolyte;
the film side of said electrode is covered in an electrode impregnated with said electrolyte to form an electrochemical couple; and
said couple is pressed while being heated to a temperature less than or equal to the temperature at which said film begins to melt, so as to obtain incomplete melting of said polymer, said electrodes becoming unseparable after cooling.
In yet another implementation of the method of the invention, both of said electrodes are used as the support, as follows:
said solution is spread in the form of a film on the surface of said porous layer of each of said electrodes;
each of said films is immersed in a volatile non-solvent that is miscible with said solvent;
said films are dried to eliminate said non-solvent;
said electrodes including said films are impregnated with said electrolyte;
said electrodes are placed together so that said films are in contact to form an electrochemical couple; and
said couple is pressed while being heated to a temperature lower than or equal to the temperature at which said film begins to melt so as to obtain incomplete melting of said polymer, said electrodes becoming uneeparable after cooling.
Using the electrodes as the support for making the polymer film makes it possible to avoid subsequent handling, and thus makes it possible to deposit films that are thinner. The method is thus simplified and made more reliable, and the final product has better performance.
Said polymer is selected from: polyvinylidene fluoride; polyvinyl chloride; polymethylmethacrylate;
cellulose acetate; a polysulfone; a polyether; a polyolefin; and from an alloy of polyvinylidene fluoride with a polymer selected from a polysulfone, polymethylmethacrylate, polyvinylpyrolidone, a copolymer of vinylidene fluoride and ethane tetrafluoride, and a copolymer of vinylacetate and of vinylalcohol.
The preferred one of said polymers is polyvinylidene fluoride (PVDF). PVDF has the advantage of exhibiting very little swelling in the presence of solvent, thus limiting dimensional changes during the manufacture of the electrochemical cell.
Said solvent is an organic solvent selected from: cyclohexanone; dichloromethane; dimethylacetamide (DMA); dimethylformamide (DMF); hexamethylphosphoramide (RMPA); dimethylaulfoxide (DMSO); triethylphosphate (TEP); N-methylpyrolidone (NMP); and mixtures thereof. It is desirable for the interaction between the polymer and the solvent to be weak.
It is preferable to use an organic solvent in which the polymer dissolves without difficulty and which can easily be eliminated by heating to a moderate temperature without running any risk of damaging the active material.
The selected polymer is put into a concentrated solution in the solvent. The concentration of the polymer must not be too high since that is one of the parameters which determines the porosity of the film; it is preferable for the solution to contain at least 50k, solvent.
The polymer in solution is deposited on the surface of the support by any known method such as immersion, coating, spraying, etc . . . . If the surface presents irregularities and a certain amount of pores, they are smoothed over by the solution and they facilitate bonding of the film.
In a first variant, the surface of said porous layer is impregnated with a wetting agent before it is covered in said film. The wetting agent may be a volatile organic solvent, for example.
In a second variant, said solution of a polymer in a solvent also contains a wetting agent at a concentration of less than 10% by weight of said polymer. The agent serves to improve penetration and distribution of the electrolyte in the polymer film.
In a third variant, said solution of a polymer in a solvent also includes a small quantity of a non-solvent, said quantity being insufficient to cause the polymer to be precipitated. The presence of a small quantity of a weak non-solvent facilitates three-dimensional organization of the polymer while it is being precipitated.
The term xe2x80x9cnon-solventxe2x80x9d is used to designate a liquid in which the polymer is not soluble (a xe2x80x9cstrongxe2x80x9d non-solvent) or is soluble to a small extent only (a xe2x80x9cweakxe2x80x9d non-solvent) at the operating temperature. When the selected non-solvent is water, either pure or in a mixture, said temperature lies in the range 5xc2x0 C. to 80xc2x0 C.
Said non-solvent is selected from: water, ethanol, ethylene-glycol, glycerol, acetone, proplyene carbonate, dichlorotnethane, ethyl acetate, butanol, pentanol, acetonitril, and mixtures thereof. If the selected non-solvent is water, the method of the invention has the advantage of not polluting the environment and of facilitating solvent recycling.
The film is put into contact with the non-solvent. The solvent is then replaced by the non-solvent with which it is miscible, thereby causing the polymer to be precipitated. Subsequent recovery of the solvent extracted by the non-solvent is thus facilitated. A porous film of solid polymer then covers the surface of the electrode. It suffices to evaporate off the non-solvent and possibly a portion of residual solvent by moderate heating.
Said electrolyte comprises a lithium salt dissolved in an organic solvent. The organic solvent comprises a mixture of ethers and/or esters, the esters being selected from linear carbonates, and cyclic carbonates having more than four carbon atoms, such as propylene, ethylene, butylene, diethyl, and dimethyl carbonates, and mixtures thereof, for example.
The lithium salt is selected from: lithium perchlorate LiCIO4, lithium hexafluoroareenate LiAsF6, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium trifluoromethanesulfonate LiCF3SO3, lithium trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), and lithium trifluoromethanesulfonmethide LiC(CF3SO2)3.
When the polymer is PVDF, heating is preferably performed at a temperature lying in the range 90xc2x0 C. to 100xc2x0 C. The coagulated polymer film is entirely stable up to a temperature of 75xc2x0 C. and it melts above 105xc2x0 C. Melting does not take place at a precise temperature, but spreads over a temperature range It starts at the beginning of the range and does not become complete until the end thereof. It is therefore preferable to operate immediately before or at the beginning of the range. It is also known that PVDP in the raw state melts at above 150xc2x0 C.
Pressing is preferably performed at a pressure lying in the range 9.81N/cm2 to 98.1N/cm2.
The present invention also provides an electrochemical cell made by the method of the invention, including an anode in which said electrochemically active material is a carbon-containing material suitable for inserting lithium in its structure and selected from graphite, coke, vitreous carbon, carbon black, and active carbon, and said polymer is polyvinylidene fluoride.
The present invention also provides an electrochemical cell made by the method of the invention, including a cathode in which said electrochemically active material is an oxide of a transition metal selected from vanadium oxide, lithium-containing oxides of manganese, nickel, and cobalt, and mixtures thereof, and said polymer is polyvinylidene fluoride.